BUILDING costs in the city are skyrocketing – with the final outlay for some public and private projects ending up double, and sometimes even triple, original estimates.

Examples abound:

* Construction of the new city courthouse on Jay Street in Brooklyn’s Metro Tech was estimated at $330 million when it was first proposed in 1992.

Today, industry insiders quietly predict the cost will exceed $700 million, and may possibly reach $1 billion, by the time the complex is completed two years from now.

* In 1992, the city decided to erect a dramatic new building – topped with a 10-story clock – to replace the fire-damaged Whitehall Ferry Terminal. The estimated cost – $112 million.

In 1998, with construction costs escalating, the design was scrapped for a simpler, two-story terminal with a $98 million price tag.

The final cost of the scaled-down terminal is now put at between $170 million and $180 million.

* Building a new New York Stock Exchange office tower and trading floor at Broad and Wall streets was estimated at $727 million last year. By last month, the estimate had zoomed to $798 million.

But with construction expected to take nearly six years, this project, too, could reach $1 billion by the time the ribbon is cut, experts say.

The reason for the meteoric price increases: the strongest construction market in a decade, coupled with the steadily rising cost of materials and skilled labor.

Also boosting costs are new – expensive – technological advances and stiffer building and safety codes.

“We are victims of the city’s success in promoting healthy development,” said Steven Fishner, who is overseeing the city’s numerous courthouse construction projects as its

criminal-justice coordinator.

City projects suffer not only from competition with private developments for materials and labor, but from bureaucratic red tape and delays – which make non-governmental work more attractive to contractors.

The booming costs of materials and labor pushed construction costs up 15 to 20 percent in the past year alone, according to Ralph Esposito of Bovis Lend Lease, one of the five largest construction companies in the world.

“Initially, the increases came in concrete, electrical and drywall. Now, the increases are across the board,” he said.

And, he noted, “Equally as challenging as the cost is the availability. You have to substitute materials because you can’t get brick for eight months.”

THEN there’s the heavy demand for skilled workers – and the city’s shrinking labor pool.

By the year 2003, there will be 30,000 unfillable positions for architects, engineers, project managers and skilled laborers, according to Louis Coletti, chairman and CEO of the Building Trades Employers’ Association, which represents 1,500 contractors.

Coletti blames the shortage on the other, more attractive jobs available today.

“Everybody wants to be a dot-com specialist or work on Wall Street,” he said.

There also are fewer training programs for the building trades than there once were. Union leaders, remembering the lean times, when their members had no work at all, have been reluctant to train too many new workers.

Coletti’s group is recruiting students from 10 of the city’s 18 vocational high schools for apprenticeship programs.

“We are looking to expand that, but it will take time and won’t address the problem we have now,” he said.

The diminishing pool of skilled labor means lots of overtime for the workers who are available.

“Rather than working seven-hour days, they are working 10-hour days, and rather than working five days a week, they are working a six- or seven-day week,” said one contractor.

That hikes costs significantly.

One of the major skill shortages is of electricians.

Since 1992, they’ve received union-negotiated wage increases of three or four percent annually, said Richard Anderson, president of the Building Congress.

“They could work 24 hours a day, seven days a week, if they wanted to,” he said.

The competition for materials and labor comes just as the city has committed $6.7 billion for capital projects -a 58.47 percent increase over last year.

New courthouses, stadiums, waste facilities, airline terminals, ferry terminals and perhaps a NYSE office tower are going up at the same time the city is repairing long-neglected roads, buildings and bridges, and investing in new public school and college facilities.

MEANWHILE, private developers – often with government incentives – are building office towers along 42nd Street, hotels from Battery Park to the airports, shopping and entertainment complexes, luxury apartments in Manhattan and more affordable housing in Harlem and the outer boroughs.

And that’s not mentioning office and home interior renovations.

All these opportunities allow contractors to be selective about the projects they go after – and sometimes results in inflated bidding on work they don’t need or want.

“It’s still very competitive, but you aren’t as hungry to get the other jobs as you would be in a slower market,” said Steven L. Sage, a vice president of Fred Geller Electrical of Long Island City.

It also enables contractors to skip government projects, which many larger firms would rather not bother with because of the long qualifying and competitive bidding processes involved and the long wait for payment.

The city has been so lax in paying contractors, it’s now required by law to pay 7.25 percent interest on late payments – with no grace period.

One of the hardest times for those working on city projects is the 90- to 120-day lag from job to payday.

“You have to be able to float a substantial amount of your working schedule … It is a problem for the small business,” said Lina Gottesman, a partner in Altus Metal & Marble Maintenance.

Most contractors prefer working in the private sector, said construction consultant Rudolph Rinaldi, a former city building commissioner.

“They’d rather go where they can make the most money, get in and out, and get paid,” he said.

Some say this is a “recipe for disaster,” said Coletti, with the city often shortchanged – getting stuck by law with barely qualified low bidders who do shoddy work because they don’t know how to do it correctly and aren’t well-supervised on the job by young, inexperienced personnel at government agencies.